At present, along with high integration and miniaturization of semiconductor devices, the heat generation amount is increasingly high, and the problem is how effectively heat is dissipated. And, a substrate which comprises a ceramic circuit plate comprising a ceramic substrate having high insulating properties and high thermal conductivity, such as an aluminum nitride substrate or a silicon nitride substrate, and a metal circuit made of copper or aluminum formed on the surface of the ceramic substrate, and which has a base plate made of copper or aluminum, as a heat dissipation plate, formed on the back side of the ceramic circuit plate, has been used e.g. as a substrate for a power module.
As a typical heat dissipation structure of a conventional ceramic circuit plate, a base plate is soldered to the ceramic circuit plate, and the base plate is commonly made of copper or aluminum. However, in such a structure, if a heat load is applied for example, cracking resulting from the difference in the coefficient of thermal expansion between the base plate and the ceramic circuit plate results in the solder layer and as a result, heat dissipation tends to be insufficient, thus leading to malfunction or fracture of the semiconductor on the circuit.
Accordingly, as a base plate having a coefficient of thermal expansion close to that of a ceramic circuit plate, an aluminum alloy-silicon carbide composite has is been proposed in JP-A-5-507030.
A base plate is bonded to a head dissipation fin, a heat dissipation unit or the like in many cases, and the shape and the bow at the joint portion are also important. For example, in a case where a base plate is bonded to a heat dissipation fin, a heat dissipation unit or the like, usually they are fixed by screws utilizing holes provided at the peripheral portion of the base plate. If there are very fine irregularities on the base plate, a gap will form between the base plate and the heat dissipation fin, the heat dissipation unit or the like, and thermal conductivity decreases in some cases even if a highly thermal conductive heat dissipating grease is applied. As a result, the heat dissipation properties of the entire module comprising the ceramic circuit plate, the base plate, and the heat dissipation fin, the heat dissipation unit or the like tend to decrease.
Accordingly, convex bow is preliminarily imparted to the base plate in many cases to prevent a gap from being formed between the base plate and the heat dissipation fin as far as possible. The bow is obtained usually by applying a pressure to the base plate with heating by using a jig having a predetermined shape. However, with respect to the bow obtained by such a method, the dispersion of the amount of bow tends to be large, and the shape of the bow is not constant, and accordingly, the quality is not stable. Further, by the dispersion of the shape of the bow, a gap may form between the base plate and the heat dissipation fin, the heat dissipation unit or the like.
There is also a method wherein the surface of the base plate is cut by machining to impart bow. However, in the case of an aluminum/silicon carbide composite, the material is very hard, and accordingly, grinding employing a tool of e.g. diamond is required, whereby the processing time tends to be long, and the cost tends to be expensive.
Accordingly, in order to solve the above problems, a method has been proposed wherein a flat silicon carbide porous body is infiltrated with a metal containing aluminum as the main component, an aluminum alloy layer is formed on both principal planes, and the aluminum alloy layer on the radiation plane side is machined. In the present invention, an aluminum alloy layer means a metal layer containing aluminum as the main component, such as an aluminum alloy layer.
However, of the base plate produced by the above method, the center portion of the aluminum alloy layer tends to be thick after machining. Accordingly, when the base plate is soldered to the ceramic circuit plate to assemble a power module, the shape at the center portion of the radiation plane may be distorted depending upon the disposition of the ceramic circuit plate, a gap is formed between the base plate and the heat dissipation fin, and no sufficient heat dissipation properties can be exhibited in some cases.
Further, in the above method, in order to control the thickness of the aluminum alloy layer on both principal planes, a high purity aluminum plate having a melting point higher than that of an aluminum alloy to be used for infiltration, is used in some cases, and local difference in the color tone may result due to the reaction between the high purity aluminum and the aluminum alloy layer having a low melting point at the time of infiltration in some cases.